Fluidic assembly of emissive displays

ABSTRACT

Fluidic assembly methods are presented for the fabrication of emissive displays. An emissive substrate is provided with a top surface, and a first plurality of wells formed in the top surface. Each well has a bottom surface with a first electrical interface. Also provided is a liquid suspension of emissive elements. The suspension is flowed across the emissive substrate and the emissive elements are captured in the wells. As a result of annealing the emissive substrate, electrical connections are made between each emissive element to the first electrical interface of a corresponding well. A eutectic solder interface metal on either the substrate or the emissive element is desirable as well as the use of a fluxing agent prior to thermal anneal. The emissive element may be a surface mount light emitting diode (SMLED) with two electrical contacts on its top surface (adjacent to the bottom surfaces of the wells).

RELATED APPLICATIONS

Any and all applications, if any, for which a foreign or domesticpriority claim is identified in the Application Data Sheet of thepresent application are hereby incorporated by reference under 37 CFR1.57.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention generally relates to integrated circuits (ICs) and, moreparticularly, to fluidic assembly method for the fabrication of emissivedisplays.

2. Description of the Related Art

The fluidic transfer of microfabricated electronic devices,optoelectronic devices, and sub-systems from a donor substrate/wafer toa large area and/or unconventional substrate provides a new opportunityto extend the application range of electronic and optoelectronicdevices. For example, display pixel size light emitting diode (LED)micro structures, such as rods, fins, or disks, can be first fabricatedon small size wafers and then be transferred to large panel glasssubstrate to make a direct emitting display. One conventional means oftransferring these LED microstructures is through a pick-and-placeprocess. However, with a display comprising millions of elements, such aprocess may take several hours to complete and is therefore inefficient.

The fluidic self-assembly of electronic devices, such as LEDs andconcentrated photovoltaics, is often performed by surface energyminimization at molten solder capillary interfaces so that bothmechanical and electrical connections can be made to an electrode duringassembly, as demonstrated in U.S. Pat. No. 7,774,929. In one aspect,electronic devices are captured in shape-matched well structures,followed by electrical integration processes, as demonstrated in U.S.Pat. No. 6,316,278.

Some problems yet to be addressed with conventional fluidic assemblyprocesses are related to the distribution method over large scales, theintegration of microcomponents to drive circuitry over large areas, andthe potential mechanisms for the repair of defective microcomponents.Over large scales, conventional fluidic assembly into wells ischallenged by the dual requirements of maximum velocities formicrocomponent capture and minimum distribution velocities forhigh-speed array assembly. Similarly, achieving the microcomponentdispense scheme and flow velocity uniformity necessary for a high yieldover the whole assembly substrate becomes very challenging overgreater-than-centimeter scales.

The integration of assembled microcomponents has been primarily done viaphotolithographically defined electrode deposition for microcomponents,or else by lamination of the second electrical contact in approacheswhere the first electrode contact is made as part of the assemblyscheme. However, the photolithography of large substrates after fluidicassembly is potentially prohibitive due to the contaminating nature ofany residual microcomponents on the substrate surface. Laminated topcontacts have not demonstrated sufficiently reliable electricalconnection to microcomponents for display applications.

Lastly, defect detection of electrically excited microcomponents is themost reliable and robust approach for inspection preceding repair.Assembled microcomponents with top-contact electrodes are at leastpartially held in an insulating matrix. Any repair that involves removalof defective microcomponents from this matrix is extremely difficult.Moreover, any similarly integrated microcomponents that are added to thearray to compensate for defective microcomponents requires that theelectrode contact process to be repeated. While technical workaroundsmay exist, they are expected to be more expensive, more time-intensive,and less reliable.

It would be advantageous if a fluidic assembly process could be used toefficiently transfer emissive elements to a display substrate with aminimum of process steps.

SUMMARY OF THE INVENTION

The fluidic assembly and orientation approach disclosed herein useshigh-variance local forcing on individual microcomponents. The highvariance in forcing results in a high variance in velocity such thatinsofar as a maximum assembly velocity exists for trapping, individualcomponent velocities may fall below that maximum threshold and settleinto wells. The second benefit to high variance is that the distributionof components over a large (meter-scale) substrate is relatively quick.Once settled into wells, the maximum forcing is such that assembledcomponents are not dislodged from a correct orientation, but misorientedcomponents are dislodged. This provides for a low-cost, high-speedassembly approach that has achieved an extrapolated assembly rate ofover 56 million devices per hour. The assembly method is a generalmethod that may be applicable to any number of substrates but is wellsuited to low-fill factor, high-area arrays with limited surfacetopography other than the wells for trapping.

Accordingly, a fluidic assembly method is provided for the fabricationof emissive displays. The method provides an emissive substrate with atop surface, and a plurality of wells formed in the top surface. Eachwell has a bottom surface with a first electrical interface, and amatrix of column and row traces forming a plurality of column/rowintersections. Each column/row intersection is associated with acorresponding well. Also provided is a liquid suspension of emissiveelements. The liquid may, for example, be an alcohol, polyol, ketone,halocarbon, or water. The method flows the suspension across theemissive substrate top surface and the emissive elements are captured inthe wells. As a result of annealing the emissive substrate, electricalconnections are made between each emissive element to the firstelectrical interface of a corresponding well. The liquid suspension mayinclude a solder fluxing agent, or the solder fluxing agent may beapplied in a separate step either prior to or subsequent to capturingthe emissive elements in the wells and prior to annealing the substrate.Additional process steps may form color modifiers and light diffusersover selected wells.

A eutectic solder interface metal on either the substrate or theemissive element is desirable as well as the use of a fluxing agentprior to thermal anneal. For example, a dimethylammonium chloride,diethanolamine, and glycerol solution may be dissolved in isopropanol.This solution can be used as the assembly fluid (suspension) or it canbe introduced after the assembly fluid is removed via sweeping andevaporation.

In some aspects, the emissive element is a surface mount light emittingdiode (SMLED) with two electrical contacts on its top surface (the SMLEDtop surfaces faces into the well, adjacent to the bottom surfaces of thewells). The electrical connections between the emissive elements and thewell first electrical interfaces are then made without the formation ofoverlying metal layers, additional conductive traces, or wire bonding onthe substrate subsequent to annealing. Otherwise, if the emissiveelements are vertical LEDs (with one electrical contact on the topsurface and one electrical contact on the bottom surface), additionalmetallization steps may be required after annealing. Typically, asemissive elements are captured in the wells, uncaptured emissiveelements are simultaneously collected and resuspended for subsequentemissive display fabrication.

In one aspect, an auxiliary mechanism is engaged for distributing theemissive elements over the substrate. Some examples of the auxiliarymechanism include a brush (rotating or non-rotating), wiper, rotatingcylinder, pressurized fluid, and mechanical vibration (e.g., acoustic orultrasonic). The auxiliary mechanism aids in the distribution, of theemissive elements across the substrate surface through contact orengagement with either the emissive elements in the suspension or withthe emissive substrate top surface. For example, assuming the emissivesubstrate has a length and a width, the method flows the suspensionacross the emissive substrate top surface at a first velocity in a firstdirection across the length of the emissive substrate. An auxiliarymechanism brush, having a rotation axis and brush length at least equalto the emissive substrate width, translates the brush length across thelength of the emissive substrate length in the first direction.Simultaneously with the first pass of brush translation, the brush isrotated to create a first local variance in the first velocity. In oneaspect, the brush rotation creates a local variance greater than thefirst velocity. The method may further repeat the brush translation inthe first direction, or in the opposite direction, and the brushrotation may create local variances either greater than or less than thefirst velocity. The brush may rotate at a rate in the range of 120 to300 revolutions per minute (RPM) and translate across the emissivesubstrate top surface at a speed in the range of 3 to 10 centimeter persecond (cm/s).

In one aspect, surface mount emissive elements are fabricated with apost extending from the bottom surface, or vertical structure emissiveelements are fabricated with a post extending from their top surface.Then, as the liquid suspension is flowed across the substrate topsurface the emissive elements move, at least partially, in response totorque created on the emissive element posts. Perhaps more important,the posts aid in capturing the emissive elements in the wells as aresult of surface orienting the emissive element top surfaces directlyoverlying the well bottom surface.

Additional details of the above-described method, as well as methods fortransferring differently shaped emissive elements to an emissivesubstrate are provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a fluidic assembly method for thefabrication of emissive displays.

FIGS. 2A and 2B are, respectively, partial cross-sectional and planviews of an exemplary emissive substrate such as might be provided inStep 102 of FIG. 1.

FIG. 3 is a partial cross-sectional view showing aspects in theperformance of Steps 104 through 108 of FIG. 1.

FIGS. 4A and 4B are, respectively, partial cross-sectional and planviews depicting an exemplary surface mount light emitting diode (SMLED).

FIG. 5 is a perspective view depicting an exemplary brush auxiliarymechanism.

FIG. 6 is a partial cross-sectional view of an emissive substrate beingpopulated by emissive elements having posts.

FIG. 7 is a flowchart illustrating a fluidic assembly method firstvariation for the fabrication of emissive displays.

FIG. 8 is a plan view supporting one exemplary version of the methoddescribed by FIG. 7.

FIG. 9 is a plan view supporting a second exemplary version of themethod described by FIG. 7.

FIG. 10 is a flowchart illustrating a fluidic assembly method secondvariation for the fabrication of emissive displays.

FIGS. 11A and 11B are, respectively, partial cross-sectional and planviews of a second exemplary emissive substrate such as might be providedin Step 102 of FIG. 1.

FIGS. 12A and 12B are partial cross-sectional views depicting thefunction of the post in emissive element surface orientation.

FIGS. 13A through 13C are partial cross-sectional views depicting theinfluence of trapping velocity on the fluidic assembly of emissiveelements.

FIG. 14 is a partial cross-sectional view depicting the effect offluidic assembly suspension fluid drag on emissive element velocityduring assembly.

DETAILED DESCRIPTION

FIG. 1 is a flowchart illustrating a fluidic assembly method for thefabrication of emissive displays. Although the method is depicted as asequence of numbered steps for clarity, the numbering does notnecessarily dictate the order of the steps. It should be understood thatsome of these steps may be skipped, performed in parallel, or performedwithout the requirement of maintaining a strict order of sequence.Generally however, the method follows the numeric order of the depictedsteps. The method starts at Step 100. Step 102 provides an emissivesubstrate.

FIGS. 2A and 2B are, respectively, partial cross-sectional and planviews of an exemplary emissive substrate such as might be provided inStep 102 of FIG. 1. The emissive substrate 200 has a top surface 202,and a first plurality of wells 204 formed in the top surface (wells204-0 through 204-2 are shown). Typically, the substrate top surface 202is planar, and the wells 204 are the only surface topographical featureaffecting fluidic assembly. Each well 204 comprises a bottom surface 206with a first electrical interface 208, which optionally may besolder-coated. First electrical interfaces 208-0 through 208-2 areshown. The emissive substrate is typically transparent and may be amultilayer structure (not shown) comprising a glass substrate with anoverlying dielectric material into which the wells have been formed. Theemissive substrate 200 also comprises a matrix of column traces 210 androw traces 212 forming a first plurality of column/row intersections214. Row traces 212-0 through 212-3 are shown, as are column/rowintersections 214-0 through 214-2. Each column/row intersection 214 isassociated with a corresponding well 204. For example, column/rowintersection 214-0 is associated with well 204-0. The column traces 210and row traces 212 may form a simple passive matrix to selectivelyenable emissive elements, or be part of an active matrix for the samepurpose, as explained in more detail below. For this reason, details ofthe interconnects between the column, rows, and electrical interfacesare not shown in this drawing.

FIGS. 11A and 11B are, respectively, partial cross-sectional and planviews of a second exemplary emissive substrate such as might be providedin Step 102 of FIG. 1. In this aspect, the emissive elements are surfacemount light emitting diodes (SMLEDs), as depicted in greater detail inFIGS. 4A and 4B. As explained below, the SMLEDs have two electricalcontacts on their top surface, which is the surface interfacing the wellbottoms 206. As a result, two electrical interfaces are formed on thewell bottoms 206; they are first electrical interfaces 208-0 through208-2 and second electrical interface 209-0 through 209-2. In thisaspect the emissive substrate 200 is formed with a passive matrix ofcolumn and row traces to selectively enable the SMLEDs. As shown, columntrace 210 is connected to the first electrical interface (208-0 through208-2) in a column of wells 204-0 through 204-2, and row traces 212-0through 212-2 are respectively connected to the second electricalinterfaces (1100-0 through 1100-2) in the column of wells 204-0 through204-2.

Returning to FIG. 1, Step 104 provides a liquid suspension of emissiveelements, and Step 106 flows the suspension across the emissivesubstrate top surface. The liquid of Step 104 may be one of a number oftypes of alcohols, polyols, ketones, halocarbons, or water. Step 108captures the emissive elements in the wells. In one aspect, Step 104provides a liquid suspension of emissive elements that includes a solderfluxing agent. Alternatively or in addition, subsequent to capturing theemissive elements in the wells (Step 108) and prior to annealing thesubstrate (Step 110), Step 109 a fills the emissive element populatedwells with a solder fluxing agent.

FIG. 3 is a partial cross-sectional view showing aspects in theperformance of Steps 104 through 108 of FIG. 1. Liquid suspension 300includes emissive elements 302, some of which are captured in wells 204,with at least an emissive element first electrical contact 304. Anemissive element second electrical contact 306 is also shown. Bothcontacts 304 and 306 are formed on the emissive element top surface 308.Likewise, a second electrical interface 310 is formed on each wellbottom surface 206.

Returning to FIG. 1, Step 110 anneals the emissive substrate. Inresponse to the annealing, Step 112 electrically connects each emissiveelement to the first electrical interface of a corresponding well. Asnoted above, the well's first electrical interface may be solder coated.Alternatively or in addition, an electrical contact or multipleelectrical contacts on the emissive elements may be solder-coated. Theannealing is performed at a high enough temperature to melt the solderbeing used.

A eutectic solder interface metal on either the substrate or theemissive element is desirable as well as the use of a fluxing agentprior to thermal anneal. Using atomic concentrations (at %), Au28/Ge62solder eutectic has a melting point (MP) of 361° C., while the meltingpoint for In49/Sn51 solder is 120° C. Pure indium is 156° C., but itsuffers from the disadvantage of being unable to bond without pressure.The fluxing agent may be a dimethylammonium chloride, diethanolamine,and glycerol solution dissolved in isopropanol, an organic acid, or arosin-type flux. This solution can be used as the assembly fluid(suspension) or it can be introduced after the assembly fluid is removedvia sweeping and evaporation.

FIGS. 4A and 4B are, respectively, partial cross-sectional and planviews depicting an exemplary surface mount light emitting diode (SMLED).The emissive elements shown in FIG. 3 may, for example, be SMLEDs. TheSMLED 302 comprises a first semiconductor layer 402, with either ann-dopant or a p-dopant. A second semiconductor layer 404 with the dopantnot used in the first semiconductor layer 402. A multiple quantum well(MQW) layer 406 is interposed between the first semiconductor layer 402and the second semiconductor layer 404. The MQW layer 406 may typicallybe a series of quantum well layers (typically 5 layers—e.g., alternating5 nm of indium gallium nitride (InGaN) with 9 nm of n-doped GaN (n-GaN))not shown. There may also be an aluminum gallium nitride (AlGaN)electron blocking layer (not shown) between MQW layers and the p-dopedsemiconductor layer. The outer layer may be p-doped GaN (Mg doping)about 200 nm thick. A high-brightness blue LED can be formed, or a greenLED if a higher indium content is used in the MQW. The most practicalfirst and second semiconductor layer materials are either galliumnitride (GaN), capable of emitting a blue or green light, or aluminumgallium indium phosphide (AlGaInP), capable of emitting red light.

In one aspect, the second electrical contact 304 is configured as aring, and the second semiconductor layer 404 has a disk shape with aperimeter underlying the second electrical contact ring. The firstelectrical contact 306 is formed within a second electrical contact 304ring perimeter, and the first semiconductor layer 402 and MQW layer 406are a stack underlying the first electrical contact. A moat may beformed between the second electrical contact 304 ring and the firstelectrical contact 306, filled with an electrical insulator 408.Additional details of the SMLED are provided in the parent applicationentitled DISPLAY WITH SURFACE MOUNT EMISSIVE ELEMENT, invented bySchuele et al., U.S. Ser. No. 15/410,001, filed Jan. 19, 2017, which isincorporated herein by reference. Advantageously, if an SMLED is used,then electrically connecting each emissive element in Step 112 includesconnecting each emissive element to the first electrical interfacewithout the formation of overlying metal layers, additional conductivetraces, wire bonding on the substrate subsequent to annealing, or theapplication of external pressure on the emissive element. In one aspectas shown, the SMLED includes a post 410 used for alignment andorientation.

More explicitly, Step 102 provides the emissive substrate with wellshaving a bottom surface with both the first electrical interface and asecond electrical interface. If a passive matrix (PM) is used, thecolumn and row traces are connected to the first and second electricalinterfaces. If an active matrix (AM) is used, the column and row tracesare used to enable a drive circuit associated with each well, whoseoutput is connected to the first electrical interface. In the case ofthe AM, the matrix of traces in the emissive substrate would furthercomprise a line connecting dc power to each drive circuit. The emissivesubstrate would also include a reference voltage network of electricalinterfaces connected to each well second electrical interface.Additional details of AM and PM enablement are provided in parentapplication Ser. No. 15/410,001.

Continuing, Step 104 provides a liquid suspension of surface mountemissive elements (e.g., SMLEDs) having a bottom surface and a topsurface, with a first electrical contact and a second electrical contactformed on the top surface. Capturing the emissive elements in the wellsin Step 108 includes capturing each surface mount emissive element topsurface directly overlying a corresponding well bottom surface.Electrically connecting each emissive element to the first electricalinterface in the corresponding well in response to the annealing (Step112) includes electrically connecting each surface mount emissiveelement first electrical contact to a corresponding well firstelectrical interface and each emissive element second electrical contactto a corresponding well second electrical interface.

In a different aspect, Step 104 provides a liquid suspension of verticalemissive elements having a bottom surface with a first electricalcontact and a top surface with a second electrical contact. Step 108captures the emissive element bottom surface directly overlying acorresponding well bottom surface, and Step 112 electrically connectseach emissive element first electrical contact to a corresponding wellfirst electrical interface. In this aspect, subsequent to electricallyconnecting the emissive element first electrical contacts tocorresponding well first electrical interfaces in Step 112, Step 114forms a reference voltage interface layer overlying the emissivesubstrate top surface. As would be understood in the art, such a stepmay entail the deposition of an isolation layer over the substrate topsurface, and etching to open contact holes through the isolation layerso that the subsequently formed reference voltage interface can beconnected to the second electrical contacts. Step 116 connects thesecond electrical contact of each vertical emissive element to thereference voltage interface layer. For example, thin-film processes canbe used to form metallization interconnects over the emissive substratetop surface. In the case of a passive matrix design using verticalemissive elements, a portion of the column/row matrix may be said to beprovided in Step 102 (e.g., the column lines) and a portion of thecolumn/row matrix (e.g., the row traces) are provided in Step 114.

In one aspect, Step 107 optionally engages an auxiliary mechanism fordistributing the emissive elements. The auxiliary mechanism may, forexample, be a brush (rotating or non-rotating), wiper, rotatingcylinder, pressurized fluid, or mechanical vibration. A “fluid” may beeither a gas or liquid. Examples of mechanical vibration includeacoustical and ultrasonic vibration. Then, Step 108 captures theemissive elements, at least partially, in response to the auxiliarymechanism engaging the emissive elements in the suspension or engagingthe emissive substrate top surface.

FIG. 5 is a perspective view depicting an exemplary brush auxiliarymechanism. Referencing FIGS. 1 and 5, Step 102 provides an emissivesubstrate 200 having a length 500 and a width 502. Step 106 supplies thesuspension at a first velocity in a first direction 504 across thelength 500 of the emissive substrate 200. Then, Step 107 engages a brush506, having a rotation axis 508 and brush length 510 at least equal tothe emissive substrate width 502 in the following substeps, Step 107 a,in a first pass, translates the brush length 510 across the emissivesubstrate length 500 in the first direction 504. In one aspect, Step 107a translates the brush at a speed in the range of 3 to 10 centimeter persecond (cm/s). Simultaneously with the first pass of brush translation,Step 107 b rotates the brush to create a first local variance in thefirst velocity. As shown, the first local variation is a greatervelocity than the first velocity. Alternatively, the first localvariation may be lesser velocity than the first velocity. In one aspect,Step 107 b rotates the brush at a rate in the range of 120 to 300revolutions per minute (RPM). In one example, the brush linear velocityat the substrate surface is 35 cm/s, and a low-velocity trapping regionoccurs in the moving front of suspension pushed out by the brush.

For example, a cylindrical brush used as an auxiliary mechanism may havean outer diameter of 50 mm and be composed of 75 micron diameter nylonor polypropylene bristles in 3 mm tufts arrayed in a close-packed,spiral pattern or two-direction spiral pattern at a 6 mm center-centertuft spacing. These dimensions are given to illustrate a cylindricalbrush that has fine, close-packed bristles made of a non-marringmaterial with desirable interactions with both microcomponents andcarrier fluid.

In one particular example the brush starts at a first edge of thesubstrate. In a first step, the brush moves to the second edge of thesubstrate, rotating counterclockwise to increase the local variance. Ina second step the brush stops a short distance from second edge, and therotation reverses to clockwise. In a third step the brush continues moveto the second edge, but then reverses translation towards first edge,still rotating clockwise. In a fourth step the brush stops a shortdistance from the first edge, and rotation reverses to counterclockwise.In a fifth step the brush finishes the translation to the first edge.Optionally, the above-described steps may be repeated.

The first velocity flow speed may be gravity-driven if the substrate istilted at an angle. The flow velocity may also oscillate or pulse. Itshould also be appreciated that the velocity of the emissive elements inthe suspension is not necessarily the same as the velocity of theliquid. As used herein, the first velocity refers to the liquidvelocity.

In one aspect, the liquid suspension disposes a high concentration of˜2-8 micron thick LEDs with diameters or maximum cross-sectionaldimensions of 20 to 150 microns suspended in isopropanol. A lowthickness of isopropanol exists over the substrate surface and ahorizontal axis brush with nylon or polypropylene bristles rotates closeto the surface. The brush is equal in length to one dimension of thesubstrate and translates across, allowing full coverage of the surface.While translating, the rotation is initially such that the linearvelocity of the bristles in contact with the liquid suspension is in thesame direction as the translation and of a higher magnitude. In thismanner, the brush forces the collection of emissive elements across thesubstrate surface. Individual emissive elements are generally movedquickly from their point of dislodgment and travel at a significantinitial velocity (similar to brush linear velocity) and travel adistance from the brush before settling on the surface again. It isgenerally this settling that allows assembly into wells.

FIGS. 13A through 13C are partial cross-sectional views depicting theinfluence of trapping velocity on the fluidic assembly of emissiveelements. At emissive element velocities (V_(O)) less than or equal to acritical trapping velocity (V_(CRIT)), the emissive elements 302 aremoving slow enough to be captured in the wells 204. The criticaltrapping velocity is represented in the initial condition for a emissiveelement approaching a trapping site well and, combined with fluiddynamics, emissive element and local substrate topography, and initialcomponent location relative to the well, defines a velocity magnitudeabove which emissive elements are not captured, and below which,emissive elements are captured. The determining factor is whether or notinteraction between the well sidewall and the emissive element providesa stopping force on the emissive element. As such, even if the majorityof the emissive element sinks below the plane of the substrate topsurface, further fluidic forcing drives the emissive element out of thewell if the emissive element leading sidewall edge is wholly above theplane of the substrate top surface. Conversely, if the leading edge ofthe emissive element is caught by the well sidewall, its momentum istransferred to the substrate and it likely settles into the well. Theconstant downward force on the emissive elements, which does not includeforcing from fluid dynamics, is the gravitational force opposed by thebuoyant force exerted by the fluid. As such, V_(CRIT) is determined byfluid density as well as by geometry and initial conditions

The critical trapping velocity is represented in the two dimensions ofthe figure while, practically, the path of emissive element travel maynot be through the well center, and thus include a component moving inor out of the two-dimensional figure. Because the drop of the emissiveelement prior to contacting the far well sidewall determines whether theemissive element is captured or not, and the on-center approachrepresents the longest path the emissive element can take withoutcontacting the sidewall, it can be appreciated that significantly lowervelocities are required to capture emissive elements that traveloff-center of the well. Put another way, the magnitude of the criticaltrapping velocity is depicted for emissive elements travelling over thecenter of the well and describes a maximum limit (to the first order) onassembly. To achieve a high yield in practice, minimum emissive elementspeeds are significantly lower than V_(CRIT) depicted here.

FIG. 14 is a partial cross-sectional view depicting the effect offluidic assembly suspension fluid drag on emissive element velocityduring assembly. At carrier fluid velocities (V) greater than a criticalcarrier fluid velocity (V_(C)), the brush 506 is likely to propel anemissive disk up, away from the surface of the substrate 202. As shown,the forces on the emissive element 302 may additionally be a function ofthe transverse velocity 1400 of the brush 506 and the rotation speed1402 of the brush. The fluid is likely to be turbulent and to someextent the travel of the emissive elements is independent of the overallfluid flow (beyond the initial brush stroke). Typically, there is a highdensity of emissive elements near the brushing region that are thenscattered forward over the substrate, flow through the fluid andexperience drag, slow-down, and finally settle on the surface and intowells before the advancing brush reaches them. So the initial velocityis necessarily very high at the brush, but the emissive elements 302slow down and settle to velocities below V_(CRIT), which is a keybenefit of the brush approach. A higher speed bristle detrapsmisoriented disks and pushes a high-density wave front of emissiveelements ahead of it, giving them a chance to settle ahead of the brush.The bristle velocity (predominantly from brush rotating speed) is chosenby the detrapping force window for oriented/misoriented disks, and thelinear travel velocity of the brush is chosen by the settling time ofthe emissive elements in liquid. In this way, this assembly approachdecouples the individual emissive element assembly speed, which islimited by V_(CRIT), from the overall display assembly speed, which isfast.

Assembly is rarely complete after a single pass, so additional passesare generally necessary with translation and rotation reversingdirection. Translation and rotation need not be reversed at the sametime, however. To conserve the population of unassembled components atopthe substrate surface (i.e., not located in wells), the rotationreverses first while the brush translates in the same direction asbefore until all unassembled components are directed back towards theassembly area—at which times the translation of the brush is reversed aswell.

In one aspect, the emissive element maximum local density at assembly inone aspect is approximately 0.3-0.8 monolayers of components to allowroom for settling with a high number of opportunities for capture. Asemissive elements are captured, it is desirable to replenish thepopulation of uncaptured (unaligned) emissive elements and fluid with anadditional dosing of suspension in front of the moving brush. Goodresults are obtained with an excess of components—that is, the number ofcomponents in liquid suspension over the assembly area exceeds thenumber of trapping sites by at least 50% to improve capture yield anddecrease assembly time. After all sites (wells) are occupied with acorrectly oriented emissive element, the excess unassembled componentsare swept off using the same brushing tool but with a different protocol(e.g. translating the brush with an extent beyond the substrate areawith uniform rotation direction). The swept-off components are collectedin a reservoir for reuse (Steps 109 a and 109 b).

A factor that distinguishes this approach is that electrical contact tocomponents does not occur during assembly or solely through depositedmetal after assembly, but rather during an anneal that exceeds theeutectic melt temperature of the emissive element-to-substrateinterfacing metal. While some prior art methods include fluxing agentssuch as HCl in aqueous suspensions for molten solder assembly, thisapproach gradually dissolves solder contacts, making consistentelectrical connection to microcomponents difficult. The concentrationsof fluxing agents used herein are initially low enough to not becaustic, but during the anneal, residual isopropanol volatilizes firstand then glycerol volatilizes. At each step, the concentration offluxing agents increases, removing surface oxides and contaminants toallow a clean metal surface for bonding. Unlike pick-and-place methods,this approach achieves good electrical contact without applying anyexternal pressure to the component interface.

In one aspect, Step 106 flows the emissive elements in the suspension,where the emissive elements have a higher percentage of occupied volumethan the liquid at the emissive substrate top surface. In a relatedvariation, Step 106 flows the suspension across the emissive substratetop surface by creating a maximum local density of emissive elements inthe suspension in the range of 0.3 to 0.8 monolayers.

FIG. 6 is a partial cross-sectional view of an emissive substrate beingpopulated by emissive elements having posts. Viewing FIGS. 1 and 6, Step104 provides a liquid suspension of emissive elements 302 with a post600 extending from surface 602. In this example the emissive element isa surface mount emissive element. Step 106 flows the liquid suspensionby moving the emissive elements across the substrate top surface, atleast partially, in response to torque created on the emissive elementposts 600. Further, capturing the emissive elements in the wells (Step108) may include surface orienting the surface mount emissive elementtop surfaces 308 directly overlying the well bottom surface in responseto the emissive element posts 600.

FIGS. 12A and 12B are partial cross-sectional views depicting thefunction of the post in emissive element surface orientation. Duringfluidic assembly, a liquid flow (indicated by arrows 1200) results indrag forces on the posts 600 of emissive elements 302 traversing thesurface of substrate 200. Because a post 600 extends from the emissiveelement surface 602, the drag forces have an asymmetric impact on thesurface orientation of the plate diodes. In particular, the drag forcesresult in a positive moment of force about a fixed point of rotation(e.g., an edge of the emissive element in contact with the surface ofsubstrate 200) that flip an inverted emissive element 302 into anon-inverted orientation. In contrast, the drag forces on a non-invertedemissive element 302 due to the liquid flow are primarily due toperturbations around post 600, and the forces exerted on the emissiveelement 302 lead to a negative net moment of force. This negative netmoment forces the leading edge (i.e., the edge leading in the directionof arrows 1200) of the emissive element down and stabilizes the emissiveelement in the non-inverted orientation.

A similar asymmetric impact of the drag forces occurs between anemissive element 302 deposited in a non-inverted orientation in well 204(see FIG. 12A), and an emissive element deposited in an invertedorientation in the well 204 (see FIG. 12B). As shown in FIG. 12A, anymoment of force around the lower right corner of emissive element 302caused by the liquid flow is offset by forces exerted on surface 602,resulting in a negative net moment of force tending to maintain theemissive element deposited in well 204. As shown in FIG. 12B, whenemissive element 302 is inverted in well 204, surface 602 acts as ahydrofoil generating a lifting force from the liquid flow such that anet positive moment of force results around the right side of emissiveelement 302 contacting the side of well 204. This net positive moment offorce tends to cause the emissive element 302 to flip in a directionindicated by an arrow 1202 such that emissive element is forced out ofthe well 204 and possibly into a non-inverted orientation as the liquidflow moves the emissive element toward another downstream well where itmay re-deposit.

In one aspect, simultaneously with capturing the emissive elements inthe wells (Step 108), Step 109 b collects the uncaptured emissiveelements, and Step 109 c resuspends the collected emissive elements forsubsequent emissive display fabrication. In another aspect, Step 118forms a plurality of color modifiers overlying the exposed surfaces of acorresponding plurality of emissive elements. Alternatively or inaddition, Step 118 forms a plurality of light diffusers overlying acorresponding plurality of emissive elements.

If the emissive element has two bottom contacts (e.g., a SMLED),annealing (Step 110) is the final processing step, save possibly colormodification integration and passivation. If electrodes are on oppositesurfaces as in the case of a vertical emissive element, a passivationlayer is deposited and opened over the emissive element top surfacecontacts and patterned metal completes electrical connection to theemissive elements (Steps 114 and 116).

FIG. 7 is a flowchart illustrating a fluidic assembly method firstvariation for the fabrication of emissive displays. The method begins atStep 700. Step 702 provides an emissive substrate with a top surface,and a plurality of wells formed in the top surface. Each well comprisesa bottom surface with a first electrical interface, and the substratefurther includes a matrix of column and row traces forming a firstplurality of column/row intersections. Each column/row intersection isassociated with a corresponding well. Step 704 provides a first liquidsuspension with a first type of emissive elements. Step 706 flows thefirst suspension across the emissive substrate top surface. Step 708captures the first type of emissive elements in the wells. Step 710provides a second liquid suspension with a second type of emissiveelements. Step 712 flows the second suspension across the emissivesubstrate top surface. Step 714 performs a final annealing of theemissive substrate. In response to the final annealing, Step 716electrically connects emissive elements to the first electricalinterface of a corresponding well. In one aspect, prior to flowing thesecond suspension, Step 709 performs an initial annealing to connect thefirst type of emissive elements to the electrical interfaces of thewells in which they have been captured. Specific details of thefabrication method can be found in the explanation of FIG. 1, above, andare not repeated here in the interest of brevity. In one aspect, thewells in which the second type of emissive elements are captured areformed after Step 708 and before Step 712.

In one aspect prior to the final annealing in Step 714, Step 713 aprovides a third liquid suspension with a third type of surface mountemissive elements. Step 713 b flows the third suspension across theemissive substrate top surface. Although not shown, an additional stepafter Step 713 b may anneal the third type of emissive elements forconnection to the electrical interfaces in the wells in which they havebeen captured. Although not shown, the method can be extended to depositany number of the emissive element types in a corresponding number ofdifferent suspensions.

FIG. 8 is a plan view supporting one exemplary version of the methoddescribed by FIG. 7. Here, Step 702 provides an emissive substrate witha plurality of circular wells 804 having a first diameter 806, and aplurality of circular wells 800 having a second diameter 802, less thanthe first diameter. Then, the first liquid suspension of Step 704provides a first type of emissive element disks 812 having a round shapewith a third diameter 814 greater than the second diameter 802 and lessthan the first diameter 806. Step 710 provides a second liquidsuspension including a second type of emissive element disks 808 havinga round shape with a fourth diameter 810 less than the second diameter802.

FIG. 9 is a plan view supporting a second exemplary version of themethod described by FIG. 7. In this aspect, Step 702 provides emissivesubstrate with a plurality of wells having a first shape 900, and aplurality of wells having a second shape 902, different than the firstshape. In this example, the first shape 900 is square and the secondshape is circular. However, the method is not limited to any particularshapes or combinations of shapes. Step 704 provides a first liquidsuspension with a first type of emissive element having the third shape904 capable of filling the first shape wells 900, but incapable offilling the second shape wells 902. Step 710 provides a second liquidsuspension with a second type of emissive element having a fourth shape906 capable of filling the second shape wells 902. In one aspect, theemissive elements with the fourth shape 906 are incapable of filling thefirst shape wells 900.

FIG. 10 is a flowchart illustrating a fluidic assembly method secondvariation for the fabrication of emissive displays. The method begins atStep 1000. Step 1002 provides an emissive substrate with a top surface,and a plurality of wells having a first shape and a plurality of wellshaving a second shape, different than the first shape. Each wellcomprises a bottom surface with a first electrical interface. Step 1002also provides a matrix of column and row traces forming a firstplurality of column/row intersections, where each column/rowintersection is associated with a corresponding well. Step 1004 providesa liquid suspension with a first type of emissive element having thethird shape capable of filling the first shape wells, but incapable offilling the second shape wells. The liquid suspension of Step 1004 alsoincludes a second type of emissive element having a fourth shape capableof filling the second shape wells, but incapable of filling the firstshape wells. Step 1006 flows the suspension across the emissivesubstrate top surface. Step 1008 captures the first type of emissiveelements in the first shape wells and the second type of emissiveelements in the second shaped wells. Step 1010 anneals the emissivesubstrate. In response to the annealing, Step 1012 electrically connectsemissive elements to the first electrical interface of a correspondingwell.

Fluidic assembly processes have been presented for the fabrication ofemissive displays. Examples of particular materials, dimensions, andcircuit layouts have been presented to illustrate the invention.However, the invention is not limited to merely these examples. Othervariations and embodiments of the invention will occur to those skilledin the art.

We claim:
 1. A fluidic assembly method for the fabrication of emissivedisplays, the method comprising: providing an emissive substrate with atop surface, a plurality of wells having a first shape and a pluralityof wells having a second shape, different than the first shape, eachwell comprising a bottom surface with a first electrical interface, andproviding a matrix of column and row traces forming a first plurality ofcolumn/row intersections, where each column/row intersection isassociated with a corresponding well; providing a liquid suspension witha first type of emissive element having a third shape capable of fillingthe first shape wells, but incapable of filling the second shape wells,and a second type of emissive element having a fourth shape capable offilling the second shape wells, but incapable of filling the first shapewells, with each first and second type of emissive element having asingle post centered on a surface; flowing the liquid suspension acrossthe emissive substrate top surface; capturing the first type of emissiveelements in the first shape wells and the second type of emissiveelements in the second shape wells, oriented so that the posts of thefirst and second type of emissive elements are exposed and extending outof respective well openings in the emissive substrate top surface;annealing the emissive substrate; and, in response to the annealing,electrically connecting the first and second type emissive elements tothe first electrical interface of corresponding wells.
 2. The method ofclaim 1 wherein electrically connecting emissive elements includesconnecting each of the first and second type emissive elements to thefirst electrical interface without the formation of overlying metallayers, additional conductive traces, or wire bonding on the substrate,subsequent to the final annealing.
 3. The method of claim 1 furthercomprising: engaging an auxiliary mechanism for distributing the firstand second type of emissive elements selected from the group consistingof a brush (rotating or non-rotating), wiper, rotating cylinder,pressurized fluid, and mechanical vibration; and, wherein capturing thefirst and second type of emissive elements in the wells includescapturing the first and second type of emissive elements in response tothe auxiliary mechanism engaging elements selected from the groupconsisting of the emissive elements in the liquid suspension and theemissive substrate top surface.
 4. The method of claim 3 whereinproviding the emissive substrate includes providing an emissivesubstrate having a length and a width; wherein engaging the auxiliarymechanism includes engaging a brush, having a rotation axis and brushlength at least equal to the emissive substrate width, as follows: in afirst pass, translating the brush length across the emissive substratelength in the first direction; simultaneously with the first pass ofbrush translation, rotating the brush to create a local variance inliquid suspension velocity.
 5. The method of claim 4 wherein engagingthe auxiliary mechanism includes: rotating the brush at a rate in arange of 120 to 300 revolutions per minute (RPM); and, translating thebrush at a speed in a range of 3 to 10 centimeter per second (cm/s). 6.The method of claim 1 wherein providing the emissive substrate includesproviding first and second shape wells with solder-coated firstelectrical interfaces.
 7. The method of claim 1 wherein flowing theliquid suspension across the emissive substrate top surface includescreating a maximum local density of first and second type emissiveelements in the liquid suspension in a range of 0.3 to 0.8 monolayers.8. The method of claim 1 wherein providing the liquid suspension offirst and second type emissive elements includes providing first andsecond type vertical emissive elements having a bottom surface with afirst electrical contact and a top surface with a second electricalcontact, with the posts centered on the top surface of the first andsecond emissive elements; wherein capturing the first and second typeemissive elements in the wells includes capturing the first and secondtype emissive element top surfaces directly overlying corresponding wellbottom surfaces; and, wherein electrically connecting the emissiveelements to the first electrical interface of a corresponding wellincludes electrically connecting each first and second type emissiveelement first electrical contact to a corresponding well firstelectrical interface.
 9. The method of claim 1 wherein electricallyconnecting the emissive elements includes electrically connecting eachfirst and second type emissive element to the first electrical interfaceof a corresponding well without the application of external pressure onthe first and second type emissive elements.
 10. The method of claim 1wherein flowing the liquid suspension across the substrate top surfaceincludes moving the first and second type emissive elements across thesubstrate top surface at least partially in response to asymmetricaldrag forces created by the surface orientation on the first and secondtype emissive elements with respect to the substrate top surface. 11.The method of claim 1 wherein capturing the first and second typeemissive elements in the wells includes surface orienting the first andsecond type emissive element first electrical contacts directlyoverlying well bottom surfaces in response to the emissive elementposts.
 12. The method of claim 1 wherein providing the emissivesubstrate includes each well comprising a bottom surface with both thefirst electrical interface and a second electrical interface; whereinproviding the liquid suspension includes providing first and second typesurface mount emissive elements having a bottom surface and a topsurface, with a first electrical contact and a second electrical contactformed on the top surface and the post centered on the bottom surface;wherein capturing the first and second type emissive elements in thewells includes capturing each first and second type surface mountemissive element top surface directly overlying a corresponding wellbottom surface; and, wherein electrically connecting the emissiveelement to the first electrical interface of a corresponding wellincludes electrically connecting each first and second type surfacemount emissive element first electrical contact to a corresponding wellfirst electrical interface and each first and second type emissiveelement second electrical contact to a corresponding well secondelectrical interface.
 13. The method of claim 1 wherein providing theliquid suspension includes providing a liquid suspension including asolder fluxing agent.
 14. The method of claim 1 further comprising:subsequent to capturing the first and second type emissive elements inthe wells, and prior to the annealing of the emissive substrate, fillingthe first and second type emissive element populated wells with a solderfluxing agent.
 15. The method of claim 1 further comprising:simultaneously with capturing the first and second type emissiveelements in the wells, collecting uncaptured first and second typeemissive elements; and, resuspending the collected first and second typeemissive elements in the liquid suspension for subsequent emissivedisplay fabrication.
 16. The method of claim 1 further comprising:forming a plurality of color modifiers overlying exposed surfaces of acorresponding plurality of emissive elements.
 17. The method of claim 1further comprising: forming a plurality of light diffusers overlying acorresponding plurality of emissive elements.
 18. The method of claim 1wherein providing the liquid suspension includes providing a liquidselected from the group consisting of alcohols, polyols, ketones,halocarbons, and water.
 19. The method of claim 1 wherein providing theliquid suspension includes providing first and second type emissiveelements having a solder-coated electrical contacts.
 20. The method ofclaim 1 wherein providing the emissive substrate includes providing aplurality of wells having a fifth shape, different than the first andsecond shapes, comprising a bottom surface with a first electricalinterface; wherein providing the liquid suspension includes providing athird type of emissive element having a sixth shape capable of fillingthe fifth shape wells, but incapable of filling the first and secondshape wells, and including a post centered on a surface; and, whereincapturing the first and second type of emissive elements includescapturing the fifth type of emissive element in the sixth shape wells,oriented so that the posts of the first and second type of emissiveelements are exposed and extending out of respective well openings inthe emissive substrate top surface.
 21. A fluidic assembly method forthe fabrication of emissive displays, the method comprising: providingan emissive substrate with a top surface, a plurality of wells having afirst shape and a plurality of wells having a second shape, differentthan the first shape, each well comprising a bottom surface with a firstelectrical interface, and providing a matrix of column and row tracesforming a first plurality of column/row intersections, where eachcolumn/row intersection is associated with a corresponding well;providing a liquid suspension with a first type of light emitting diode(LED) having a third shape capable of filling the first shape wells, butincapable of filling the second shape wells, and a second type of LEDhaving a fourth shape capable of filling the second shape wells, butincapable of filling the first shape wells, wherein the first and secondtype of LEDs each have a post extending from a bottom surface; flowingthe liquid suspension across the emissive substrate top surface;capturing the first type of LEDs in the first shape wells, surfaceorienting the first type of LED top surfaces directly overlying thefirst shape well bottom surfaces with the posts exposed and extendingout of first shape well openings in the emissive substrate top surfacein response to the first type of LED posts, and capturing the secondtype of LED in the second shape wells, surface orienting the second typeof LED top surfaces directly overlying the second shape well bottomsurfaces with the posts exposed and extending out of second shape wellopenings in the emissive substrate top surface in response to the secondtype of LED posts; annealing the emissive substrate; and, in response tothe annealing, electrically connecting the first and second type LEDs tothe first electrical interface of corresponding wells.
 22. A fluidicassembly method for the fabrication of emissive displays, the methodcomprising: providing an emissive substrate with a top surface, aplurality of wells having a first shape and a plurality of wells havinga second shape, different than the first shape, each well comprising abottom surface with a first electrical interface, and providing a matrixof column and row traces forming a first plurality of column/rowintersections, where each column/row intersection is associated with acorresponding well; providing a liquid suspension comprising a firsttype of vertical emissive element having a third shape capable offilling the first shape wells, a top surface with a first electricalcontact, a post extending from the top surface, and a bottom surfacewith a second electrical contact, wherein the first type of verticalemissive element is incapable of filling the second shape wells, and theliquid suspension also comprising a second type of emissive elementhaving a fourth shape capable of filling the second shape wells, a topsurface with a first electrical contact, a post extending from the topsurface, and a bottom surface with a second electrical contact, whereinthe second type of vertical emissive element is incapable of filling thefirst shape wells; flowing the liquid suspension across the emissivesubstrate top surface; capturing the first type of emissive elements inthe first shape wells and the second type of emissive elements in thesecond shape wells with the posts exposed and extending out ofrespective well openings in the emissive substrate top surface;annealing the emissive substrate; and, in response to the annealing,electrically connecting the first and second type emissive elements tothe first electrical interface of corresponding wells.
 23. A fluidicassembly method for the fabrication of emissive displays, the methodcomprising: providing an emissive substrate with a top surface, aplurality of wells having a first shape and a plurality of wells havinga second shape, different than the first shape, each well comprising abottom surface with a first electrical interface, and providing a matrixof column and row traces forming a first plurality of column/rowintersections, where each column/row intersection is associated with acorresponding well; providing a liquid suspension with a first type ofvertical emissive element having a third shape capable of filling thefirst shape wells, but incapable of filling the second shape wells, anda second type of vertical emissive element having a fourth shape capableof filling the second shape wells, but incapable of filling the firstshape wells; wherein the first and second type vertical emissiveelements each have a bottom surface with a first electrical contact anda top surface with a second electrical contact, with a post centered onthe top surface; flowing the liquid suspension across the emissivesubstrate top surface; capturing the top surface of first type ofvertical emissive elements in the first shape wells and the top surfaceof the second type of vertical emissive elements in the second shapewells, directly overlying corresponding well bottom surfaces; annealingthe emissive substrate; and, in response to the annealing, electricallyconnecting the first electrical contact of the first and second typevertical emissive elements to the first electrical interface ofcorresponding wells.
 24. A fluidic assembly method for the fabricationof emissive displays, the method comprising: providing an emissivesubstrate with a top surface, a plurality of wells having a first shapeand a plurality of wells having a second shape, different than the firstshape, each well comprising a bottom surface with a first electricalinterface and a second electrical interface, and providing a matrix ofcolumn and row traces forming a first plurality of column/rowintersections, where each column/row intersection is associated with acorresponding well; providing a liquid suspension with a first type ofsurface mount emissive element having a third shape capable of fillingthe first shape wells, but incapable of filling the second shape wells,and a second type of surface mount emissive element having a fourthshape capable of filling the second shape wells, but incapable offilling the first shape wells; wherein the first and second type surfacemount emissive elements each have a bottom surface and a top surface,with a first electrical contact and a second electrical contact formedon the top surface, and a post centered on the bottom surface; flowingthe liquid suspension across the emissive substrate top surface;capturing the top surface of the first type of surface mount emissiveelements directly overlying the bottom surface of the first shape wellsand capturing the top surface of the second type of surface mountemissive elements directly overlying the bottom surface of the secondshape wells; annealing the emissive substrate; and, in response to theannealing, electrically connecting the first and second electricalcontacts of the first and second type of surface mount emissiveelements, respectively, to the first and second electrical interfaces ofcorresponding wells.